In-circuit testing apparatus are known in the art. See, for example, copending application Ser. No. 175,713 filed 3/31/88, issued 12/14/89 as U.S. Pat. No. 488854 entitled "Programmatically Generated In-Circuit Test of Digital to Analog Converters" and copending application Ser. No. 175,874 filed 3/31/88, issued 8/7/90 as U.S. Pat. No. 4,947,106 entitled "Programmatically Generated In-Circuit Test of Analog to Digital Converters." See also U.S. Pat. No. 4,797,627. As is known, in-circuit testing involves electrically isolating the device under test ("DUT") from the remainder on the devices on the circuit board under test ("BUT"), then performing "pinchecks" and/or "functionality tests" on the DUT.
A common method of electrically isolating a DUT is to overdrive the DUT's inputs with externally generated signals of greater magnitude than those generated on the BUT. The externally generated overriding signals are of known voltage, frequency, etc., and, when they successfully override the signals generated in-circuit, the DUT responds to the externally generated signal rather than to the in-circuit generated signal. If overdriving is successful, then the DUT output can be accurately compared to an expected response by the in-circuit tester. However, if overdriving is not successful, or if the overdriving signal combines with or adds to the in-circuit generated signal, then the actual input to the DUT is unknown and hence the output cannot be accurately compared to an expected response. This is particularly a problem with in-circuit testers that have overdrive current limitations and in-circuit testers that are incapable of synchronizing their overdrive signals with those to be overdriven on the BUT.
Consider the example of FIG. 1. High current, high frequency clock pulses from an asynchronous can-type oscillator 12 on the BUT 10 directly drive a plurality of digital DUT's 14. As shown in FIG. 2, an attempt to overdrive the oscillator's clock pulses 16 with externally supplied pulses 18 from an in-circuit tester, such as a Hewlett-Packard Model 3065, results in an asynchronous combined clock signal 20 since the two clock signals are not synchronized. The combined clock signal 20 has variable amplitude and is severely distorted, even though the externally and internally generated clock signals are each bilevel. Applying a signal like that indicated by 20 to the clock inputs of the DUT's 14 would not produce any predictable output.
It is therefore desirable to provide an apparatus and method for use with an in-circuit tester that predictably overdrives the in-circuit generated signal and which is simple and inexpensive to implement. The present invention achieves these goals.